Liquefier assembly for use in extrusion-based additive manufacturing systems

ABSTRACT

A liquefier assembly for use in an extrusion-based additive manufacturing system, the liquefier assembly comprising a downstream portion having a first average inner cross-sectional area, and an upstream having a second average inner cross-sectional area that is less than the first inner cross-sectional area, the upstream portion defining a shoulder configured to restrict movement of a melt meniscus of a consumable material.

CROSS-REFERENCE TO RELATED APPLICATION(S)

Reference is hereby made to U.S. patent application Ser. No. ______,filed on even date, and entitled “Method For Building Three-DimensionalModels With Extrusion-Based Additive Manufacturing Systems” (attorneydocket no. S697.12-0180), the disclosure of which is incorporated byreference.

BACKGROUND

The present disclosure relates to additive manufacturing systems forbuilding three-dimensional (3D) models with layer-based additivemanufacturing techniques. In particular, the present disclosure relatesto liquefier assemblies for use in extrusion-based additivemanufacturing systems.

An extrusion-based additive manufacturing system (e.g., fused depositionmodeling systems developed by Stratasys, Inc., Eden Prairie, Minn.) isused to build a 3D model from a digital representation of the 3D modelin a layer-by-layer manner by extruding a flowable modeling material.The modeling material is extruded through an extrusion tip carried by anextrusion head, and is deposited as a sequence of roads on a substratein an x-y plane. The extruded modeling material fuses to previouslydeposited modeling material, and solidifies upon a drop in temperature.The position of the extrusion head relative to the substrate is thenincremented along a z-axis (perpendicular to the x-y plane), and theprocess is then repeated to form a 3D model resembling the digitalrepresentation.

Movement of the extrusion head with respect to the substrate isperformed under computer control, in accordance with build data thatrepresents the 3D model. The build data is obtained by initially slicingthe digital representation of the 3D model into multiple horizontallysliced layers. Then, for each sliced layer, the host computer generatesa build path for depositing roads of modeling material to form the 3Dmodel.

In fabricating 3D models by depositing layers of a modeling material,supporting layers or structures are typically built underneathoverhanging portions or in cavities of objects under construction, whichare not supported by the modeling material itself. A support structuremay be built utilizing the same deposition techniques by which themodeling material is deposited. The host computer generates additionalgeometry acting as a support structure for the overhanging or free-spacesegments of the 3D model being formed. Support material is thendeposited from a second nozzle pursuant to the generated geometry duringthe build process. The support material adheres to the modeling materialduring fabrication, and is removable from the completed 3D model whenthe build process is complete.

SUMMARY

A first aspect of the present disclosure is directed to a liquefierassembly for use in an extrusion-based additive manufacturing system.The liquefier assembly includes a downstream portion having a first endand a second end opposite of the first end along a longitudinal lengthof the liquefier assembly, and a first average inner cross-sectionalarea. The liquefier assembly also includes an upstream portion disposedadjacent to the first end of the downstream portion, where the upstreamportion is configured to receive a consumable material and has a secondaverage inner cross-sectional area that is less than the first innercross-sectional area. The upstream portion also defines a shoulderbetween the first inner cross-sectional area and the second innercross-sectional area, where the shoulder is configured to restrictmovement of a melt meniscus of the consumable material. The liquefierassembly further includes an extrusion tip disposed at the second end ofthe downstream portion.

Another aspect of the present disclosure is directed to a liquefierassembly for use in an extrusion-based additive manufacturing system,where the liquefier assembly includes a liquefier tube, a hollow liner,and an extrusion tip. The liquefier tube has an inlet end, an outletend, and an inner surface, where the inner surface of the liquefier tubedefines a first average inner cross-sectional area. The hollow liner isdisposed at least partially within the liquefier tube such that anoutlet end of the hollow liner is disposed within the liquefier tube.The hollow liner has an inner surface that defines a second averageinner cross-sectional area, where the first average innercross-sectional area is greater than the second average innercross-sectional area. The extrusion tip disposed at the outlet end ofthe liquefier tube at an offset location from the outlet end of thehollow liner.

Another aspect of the present disclosure is directed to a liquefierassembly for use in an extrusion-based additive manufacturing system,where the liquefier assembly includes a downstream portion and anupstream portion. The downstream portion compositionally includes athermally-conductive material, and has an inner surface with a firstaverage inner cross-sectional area. The upstream portion is disposedadjacent to the downstream portion, and has an low-stick inner surfaceand a second average inner cross-sectional area that is less than thefirst average inner cross-sectional area. The liquefier assembly alsoincludes an extrusion tip disposed at an opposing end of the downstreamportion from the upstream portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of an extrusion-based additive manufacturingsystem that includes a stepped liquefier assembly of the presentdisclosure.

FIG. 2A is a side sectional view of a non-stepped liquefier assembly inuse with a filament.

FIG. 2B is a side sectional view of the non-stepped liquefier assemblyin use with the filament, which illustrates a meniscus dry down effectdue to an increased filament feed rate.

FIG. 2C is a side sectional view of the non-stepped liquefier assemblyin use with the filament, which illustrates a raised meniscus due tolatent heating and thermal expansion.

FIG. 3 is a side sectional view of a stepped liquefier assembly of thepresent disclosure, which includes a hollow liner.

FIG. 4 is a side sectional view of the stepped liquefier assembly of thepresent disclosure in use with a filament.

FIG. 5 is a side sectional view of a first alternative stepped liquefierassembly of the present disclosure in use with a filament, whichincludes a coated hollow liner.

FIG. 6 is a side sectional view of a second alternative steppedliquefier assembly of the present disclosure in use with a filament,which includes a thin liquefier tube wall and a thermal sheath.

FIG. 7 is a side sectional view of a third alternative stepped liquefierassembly of the present disclosure in use with a filament, whichincludes a thin liquefier tube wall, a thermal sheath, and a lower tube.

FIG. 8 is a side sectional view of a fourth alternative steppedliquefier assembly of the present disclosure in use with a filament,which includes a thin liquefier tube wall, a thermal sheath, a threadedhollow liner, and a rotary drive nut.

FIG. 9 is a side sectional view of a fifth alternative stepped liquefierassembly of the present disclosure in use with a filament, whichincludes a sensor retained between a hollow liner and a liquefier tube.

FIG. 10 is a side sectional view of a sixth alternative steppedliquefier assembly of the present disclosure in use with a filament,which includes a sloped shoulder.

FIG. 11 is an expanded view of section 11 taken in FIG. 10, furtherillustrating the sloped shoulder.

FIG. 12A is a sectional view of section 12A-12A taken in FIG. 3,illustrating a hollow liner having an annular cross-sectional geometry.

FIG. 12B is a first alternative sectional view of section 12A-12A takenin FIG. 3, illustrating a hollow liner having a non-annularcross-sectional geometry with a pair of rectangular apertures.

FIG. 12C is a second alternative sectional view of section 12A-12A takenin FIG. 3, illustrating a hollow liner having a non-annularcross-sectional geometry with multiple circular apertures.

FIG. 12D is a third alternative sectional view of section 12A-12A takenin FIG. 3, illustrating a hollow liner having a non-annularcross-sectional geometry with an off-axis aperture.

FIG. 13 is a sectional view of a seventh alternative stepped liquefierassembly of the present disclosure, which includes a liquefier tuberetained within a thermal sheath.

DETAILED DESCRIPTION

The present disclosure is directed to a stepped liquefier assembly foruse in extrusion-based additive manufacturing systems. The steppedliquefier assembly includes an upstream portion and a downstreamportion, where the upstream portion has a smaller inner cross-sectionalarea than the downstream portion. As discussed below, this steppedcross-sectional areas restricts movement of a meniscus of a moltenfilament material to a desired height within the stepped liquefierassembly, thereby improving control of the flow rate through the steppedliquefier assembly. The improved flow rate control is desirable forbuilding 3D models and support structures having good part resolutionand reduced build times.

As shown in FIG. 1, system 10 is an exemplary extrusion-based additivemanufacturing system for building 3D models, and includes build chamber12, platen 14, gantry 16, extrusion head 18, and supply sources 20 and22. Examples of suitable systems for system 10 include extrusion-basedadditive manufacturing systems, such as fused deposition modelingsystems developed by Stratasys, Inc., Eden Prairie, Minn. As discussedbelow, extrusion head 18 may include one or more stepped liquefierassemblies of the present disclosure (not shown in FIG. 1) for meltingsuccessive portions of filaments (not shown in FIG. 1) during a buildoperation with system 10.

Build chamber 12 contains platen 14, gantry 16, and extrusion head 18for building a 3D model (referred to as 3D model 24) and optionally acorresponding support structure (referred to as support structure 26).Platen 14 is a platform on which 3D model 24 and support structure 26are built, and desirably moves along a vertical z-axis based on signalsprovided from computer-operated controller 28. Gantry 16 is a guide railsystem that is desirably configured to move extrusion head 18 in ahorizontal x-y plane within build chamber 12 based on signals providedfrom controller 28. The horizontal x-y plane is a plane defined by anx-axis and a y-axis (not shown in FIG. 1), where the x-axis, the y-axis,and the z-axis are orthogonal to each other. In an alternativeembodiment, platen 14 may be configured to move in the horizontal x-yplane within build chamber 12, and extrusion head 18 may be configuredto move along the z-axis. Other similar arrangements may also be usedsuch that one or both of platen 14 and extrusion head 18 are moveablerelative to each other.

Extrusion head 18 is supported by gantry 16 for building 3D model 24 andsupport structure 26 on platen 14 in a layer-by-layer manner, based onsignals provided from controller 28. In the embodiment shown in FIG. 1,extrusion head 18 is a dual-tip extrusion head configured to depositmodeling and support materials from supply source 20 and supply source22, respectively. Examples of suitable extrusion heads for extrusionhead 18 include those disclosed in Crump et al., U.S. Pat. No.5,503,785; Swanson et al., U.S. Pat. No. 6,004,124; LaBossiere, et al.,U.S. Pat. No. 7,604,470; and Leavitt, U.S. Pat. No. 7,625,200.Furthermore, system 10 may include a plurality of extrusion heads 18 fordepositing modeling and/or support materials from one or more tips.

The modeling material is supplied to extrusion head 18 from supplysource 20 via feed line 30, thereby allowing extrusion head 18 todeposit the modeling material to build 3D model 24. Correspondingly, thesupport material is supplied to extrusion head 18 from supply source 22via feed line 32, thereby allowing extrusion head 18 to deposit thesupport material to build support structure 26. During a buildoperation, gantry 16 moves extrusion head 18 around in the horizontalx-y plane within build chamber 12, and one or more drive mechanisms,such as drive mechanisms 34 and 36, are directed to intermittently feedthe modeling and support materials through extrusion head 18 from supplysources 20 and 22. Examples of suitable drive mechanisms for drivemechanisms 34 and 36 include those disclosed in Crump et al., U.S. Pat.No. 5,503,785; Swanson et al., U.S. Pat. No. 6,004,124; LaBossiere, etal., U.S. Pat. Nos. 7,384,255 and 7,604,470; Leavitt, U.S. Pat. No.7,625,200; and Batchelder et al., U.S. Patent Application PublicationNo. 2009/0274540.

The received modeling and support materials are then deposited ontoplaten 14 to build 3D model 24 and support structure 26 using alayer-based additive manufacturing technique. Support structure 22 isdesirably deposited to provide vertical support along the z-axis foroverhanging regions of the layers of 3D model 24, allowing 3D model 24to be built with a variety of geometries. After the build operation iscomplete, the resulting 3D model 24/support structure 26 may be removedfrom build chamber 12. Support structure 26 may then be removed from 3Dmodel 24.

The consumable materials may be provided to system 10 in anextrusion-based additive manufacturing system in a variety of differentmedia. Commonly, the material is supplied in the form of a continuousfilament. For example, in system 10, the modeling and support materialsmay be provided as continuous filament strands fed respectively fromsupply sources 20 and 22, as disclosed in Swanson et al., U.S. Pat. No.6,923,634; Comb et al., U.S. Pat. No. 7,122,246; and Taatjes et al, U.S.Patent Application Publication Nos. 2010/0096485 and 2010/0096489.Examples of suitable average diameters for the filament strands of themodeling and support materials range from about 1.27 millimeters (about0.050 inches) to about 3.0 millimeters (about 0.120 inches). The terms“about” and “substantially” are used herein with respect to measurablevalues and ranges due to expected variations known to those skilled inthe art (e.g., limitations and variabilities in measurements).Additionally, the terms “three-dimensional model” and “3D model” referto objects, parts, and the like built using layer-based additivemanufacturing techniques, and are not intended to be limited to anyparticular use.

FIGS. 2A-2C illustrate liquefier assembly 38 in use with filament 40. Asshown in FIG. 2A, liquefier assembly 38 includes cylindrical liquefiertube 42 and extrusion tip 44, and is shown melting and extruding thematerial of filament 40. Liquefier assembly 38 does not include astepped cross-sectional area, but is otherwise suitable for use in anextrusion-based additive manufacturing system for building high-quality3D models and support structures. In the shown example of FIGS. 2A-2C,filament 40 may be driven downward with a filament drive mechanism ofthe extrusion-based additive manufacturing system (e.g., drive mechanism34, shown in FIG. 1).

Filament 40 is heated to be flowable in liquefier assembly 38, while atthe same time a portion of filament 40 entering liquefier assembly 38remains solid. The strand of filament 40 acts like a piston, andresulting pressurization impels molten material out of extrusion tip 44.The flow rate of material extruded from extrusion tip 44 is controlledby the rate at which filament 40 enters liquefier assembly 38. Thematerial is deposited from extrusion tip 44 in “roads” according to toolpaths generated from design data, and the deposited material solidifiesto form the model. Any deviations between the deposited roads and adesired extrusion profile may impair the model quality. To build a 3Dmodel which accurately matches the design data, it is thus desirable toaccurately predict the flow rate of extruded material.

FIGS. 2A-2C illustrate the counteracting mechanisms that may occurwithin liquefier assemblies, such as meniscus dry down effects, latentheating and material expansion, and filament diameter variations, eachof which may alter the extrusion rates and flow controls of theliquefier assemblies, potentially resulting in inferior model quality.

Liquefier assembly 38 includes a heated lower longitudinal region,referred to as zone 46. Liquefier tube 42 at zone 46 correspondinglyheats and melts the material of filament 40 to form melt 48. The regionabove zone 46, referred to as zone 50, is not directly heated such thata thermal gradient is formed along the longitudinal length of liquefiertube 42.

The molten portion of the filament material (i.e., melt 48) formsmeniscus 52 around the unmelted portion of filament 40. While operatingat steady state, liquefier assembly 38 has its maximum flow ratedictated by its heated length and the thermal diffusivity of thefilament material being extruded. As shown in FIG. 2A, during asteady-steady extrusion of the material through extrusion tip 44, theheight of meniscus 52 is maintained at nominal height 54 along thelongitudinal length of liquefier tube 42. The downward movement offilament 40 functions as a viscosity pump to extrude the material inmelt 48 out of extrusion tip as extruded road 56. The hydraulic pressurerequired to extrude the material out of extrusion tip 44 is generated byshearing the flow of the molten material within a few millimeters ofmeniscus 52 using the viscosity pump action.

As shown in FIG. 2B, when the feed rate of filament 40 is increased, theheight of meniscus 52 drops toward extrusion tip 44, such as to height58. At first glance, this drop in height of meniscus 52, referred toherein as “dry down”, appears to be counterintuitive. However, the drydown of meniscus 52 from nominal height 54 (shown in FIG. 2A) to height58 occurs primarily due to two complimentary mechanisms. First, thepressure generated by a constant length of the viscosity pump actionbelow meniscus 52 increases linearly with the shear rate. Second, thelongitudinal length of the viscosity pump action below meniscus 52increases with the feed rate of filament 40 because the time required tomelt filament 40 in an inward axial direction from its outer surface isconstant.

Therefore, the height of meniscus 52 is an unstable equilibrium, wheretwo mechanisms counteract the vertical motion of meniscus 52. First, asmeniscus 52 moves downward towards extrusion tip 44 and the flow rateincreases, the average viscosity of melt 48 flowing through extrusiontip 44 increases, thereby requiring extra pressure to maintain the flow.Additionally, when the flow of the extruded material used to form road56 is greater than an amount that will fit between extrusion tip 44 andthe 3D model or the support structure, the extruded material generatesan upward backpressure.

Even small changes in the height of meniscus 52 can have substantialeffects on the quality of the 3D model or support structure being built.As an example, for liquefier assembly 38 having cylindrical liquefiertube 42 with an inner diameter of 1.88 millimeters (0.074 inches) andwith filament 40 having an average diameter of 1.78 millimeters (0.070inches) (i.e., the gap is about 0.102 millimeters (about 0.004 inches)),a change in height of meniscus 52 of one inch may produce extruded road56 having a length of about 250 millimeters (about 10 inches), a heightof about 0.127 millimeters (about 0.005 inches), and a width of about0.254 millimeters (about 0.010 inches), with no change in the positionof filament 40.

Additionally, as shown in FIG. 2C, in the event that liquefier assembly38, while operating at a steady-state extrusion rate at a modest speed,is suddenly directed to stop extruding, latent heat may continue to meltthe filament material within liquefier tube 42. This molten materialexpands, thereby pushing meniscus 52 upward into zone 50 of liquefiertube 42, such as to height 60. The cooler temperature of zone 50 maythen solidify the molten material within zone 50, thereby effectivelyplugging up liquefier assembly 38.

Furthermore, liquefier assembly 38 maybe susceptible to variations inthe diameter of filament 40 over the length of filament 40. As thediameter of filament 40 increases, the viscosity pump action belowmeniscus 52 becomes more efficient due to the decrease in gap 56 betweenthe outer surface of filament 40 and the inner surface of liquefier tube42. As a result, the viscosity pump action pushes meniscus 52 downward.Alternatively, as the diameter of filament 40 decreases, the viscositypump action below meniscus 52 becomes less efficient, resulting inmeniscus 52 rising upward.

As shown in FIGS. 2A-2C, the extrusion properties of liquefier assembly38 require a balance of counteracting mechanisms, such as meniscus drydown effects, latent heating and material expansion, and filamentdiameter variations. These counteracting mechanisms may balance outduring steady-state operation at a particular feed rate of filament 40.However, when the feed rate of filament 40 increases or decreases, or ifthe diameter of filament 40 fluctuates, the height of meniscus 52 mayvary. This may alter the extrusion rate and response time of liquefierassembly 38. Thus, as discussed below, restricting movement of themeniscus of the molten filament material is desirable to improve controlover extrusion rates and response times, thereby improving part qualityand build times.

FIGS. 3 and 4 illustrate liquefier assembly 62, which is an example of astepped liquefier assembly of the present disclosure, and is a suitableliquefier assembly for use in system 10 (shown in FIG. 1). As discussedbelow, liquefier assembly 62 is capable of restricting movement of ameniscus at a desired height. In particular, liquefier assembly 62achieves a combination of reducing meniscus dry down effects, blockingmolten material from flowing upward due to thermal expansion, andcompensating for filament diameter variations. This improves control ofextrusion rates and response times for building 3D models and supportstructures (e.g., 3D model 24 and support structure 26).

As shown in FIG. 3, liquefier assembly 62 includes liquefier tube 64,extrusion tip 66, and hollow liner 68, where the location of hollowliner 68 divides liquefier assembly 62 into upstream portion 62 a anddownstream portion 62 b. As used herein, the terms “upstream” and“downstream” refer to locations relative to a direction of movement of amaterial through the liquefier assembly (e.g., in a downward directionin the view shown in FIG. 3). As discussed below, upstream portion 62 ahas a smaller inner diameter than downstream portion 62 b, therebyproviding a stepped cross-sectional area.

Liquefier tube 64 includes inlet end 70 and outlet end 72, which areoffset along longitudinal axis 74. Liquefier tube 64 also includes innersurface 76, which, in the shown embodiment in which liquefier tube 64and hollow liner 68 are cylindrical, is an inner diameter surface ofliquefier tube 64 and downstream portion 62 b. While liquefier assembly62 and hollow liner 68 are discussed herein as having cylindricalgeometries, liquefier assemblies of the present disclosure mayalternatively include non-cylindrical geometries. Accordingly, as usedherein unless otherwise indicated, the terms “tube” and “hollow liner”include a variety of hollow geometries, such as cylindrical geometries,elliptical geometries, polygonal geometries (e.g., rectangular andsquare geometries), axially-tapered geometries, and the like.

Liquefier tube 64 functions as a thin-wall liquefier for transferringheat from one or more external heat transfer components, such as heatingcoil 77, to a filament (not shown in FIG. 3) retained within liquefiertube 64. Heating coil 77 desirably extends around a lower portion ofliquefier tube 64 and/or extrusion tip 66 to define zone 78 for heatingand melting the filament. In particular, heating coil 77 heats liquefiertube 64 at zone 78, and liquefier tube 64 correspondingly conducts theheat to melt the modeling or support material of the filament withinzone 78. The region above zone 78, referred to as zone 80, is desirablynot directly heated by heating coil 77 such that a thermal gradient isformed along longitudinal length 74 of liquefier tube 64.

Heating coil 77 is one or more thermally-conductive coils or wires(e.g., nickel-chromium wire) wrapped around a portion of liquefier tube64 and/or extrusion tip 66. Liquefier assembly 62 may also include oneor more thermal-spreading sheaths (e.g. aluminum sleeves) (not shown)between liquefier tube 64 and heating coils 77, and one or moreinsulation sleeves (not shown) wrapped around heating coils 77. In oneembodiment, the heating coil 77 may include a multiple-zone arrangementas disclosed in Swanson et al., U.S. patent application Ser. No.12/841,341, entitled “Multiple-Zone Liquefier Assembly forExtrusion-Based Additive Manufacturing Systems”. Examples of suitablealternative heat transfer components for use with liquefier assembly 62include those disclosed in Swanson et al., U.S. Pat. No. 6,004,124;Comb, U.S. Pat. No. 6,547,995; LaBossiere et al., U.S. Publication No.2007/0228590; and Batchelder et al., U.S. Patent Application PublicationNo. 2009/0273122.

Liquefier tube 64 is fabricated from one or more thermally-conductivematerials to transfer thermal energy from the external heat transfercomponent(s) to the filament. Suitable materials for liquefier tube 64include materials having average thermal conductivities greater thanabout 10 watts/meter-Kelvin (W/m-K), where thermal conductivitiesreferred to herein are measured pursuant to ASTM E1225-09. Additionally,the material for liquefier tube 64 is desirably capable of withstandingthe thermal environment of build chamber 12 (shown in FIG. 1) andliquefier assembly 62 without melting or thermal degradation for asuitable operational life. Examples of suitable materials for liquefiertube 64 include metallic materials (e.g., stainless steel), graphite,and the like.

In the shown embodiment, liquefier tube 64 has a substantially uniformwall thickness, referred to as wall thickness 64 t. Examples of suitableaverage thicknesses for wall thickness 64 t range from about 0.1millimeters (about 0.004 inches) to about 0.3 millimeters (about 0.12inches), with particularly suitable thicknesses ranging from about 0.15millimeters (about 0.006 inches) to about 0.22 millimeters (about 0.009inches). Suitable lengths for liquefier tube 64 along longitudinal axis74 may vary depending on desired processing conditions, such that thelength of liquefier tube 64 is sufficient for retaining hollow liner 68and for providing zone 78.

In the shown embodiment, liquefier tube 64 also has a substantiallyuniform inner diameter at inner surface 76, referred to as innerdiameter 76 d, which is also the inner diameter of downstream portion 62b. Examples of suitable average dimensions for inner diameter 76 d rangefrom about 1.40 millimeters (about 0.055 inches) to about 3.18millimeters (about 0.125 inches), where the average dimensions may bedesigned in combination with the dimensions of an intended filamentdiameter. Accordingly, in some embodiments, examples of suitabledimensions for inner diameter 76 d range from about 105% to about 150%of the average diameter of the intended filament, with particularlysuitable dimensions ranging from about 110% to about 125% of the averagediameter of the intended filament.

The use of hollow liner 68 allows inner diameter 76 d of liquefier tube64 to be greater than a corresponding inner diameter of liquefier tube38 (shown in FIGS. 2A-2C), while still maintaining a good viscosity pumpaction. In other words, as discussed below, the gap between innersurface 76 of liquefier tube 64 and an average diameter of a filamentused with liquefier assembly 62 is desirably greater than the gapbetween an inner surface of liquefier tube 38 and the average diameterof filament 40 (shown in FIGS. 2A-2C).

Extrusion tip 66 is a small-diameter tip that is secured to outlet end72 of liquefier tube 64 and is configured to extrude molten material ata desired road width. Examples of suitable inner tip diameters forextrusion tip 66 range from about 125 micrometers (about 0.005 inches)to about 510 micrometers (about 0.020 inches). Extrusion tip 66 may befabricated from one or more materials configured to withstand thethermal environment of build chamber 12 (shown in FIG. 1) and liquefierassembly 62 without melting or thermal degradation for a suitableoperational life, such as one or more metallic materials (e.g.,stainless steel). Extrusion tip 66 may be secured to liquefier tube 64at outlet end 72 in a variety of manners, such as snap-based,fastener-based, or screw-based mechanical interlocking, welding, and thelike.

Hollow liner 68 is located at least partially within liquefier tube 64,and desirably extends around inner surface 76 above zone 78. Hollowliner 68 includes inlet end 82 and outlet end 84, which are offset alonglongitudinal axis 74. Hollow liner 68 also include inner surface 86extending between inlet end 82 and outlet end 84, which is also theinner surface of upstream portion 62 a. In the shown embodiment, inletend 82 is adjacent to inlet end 70 of liquefier tube 64. In alternativeembodiments, inlet end 82 may extend further above inlet end 70 alonglongitudinal axis 76.

As discussed above, the location of hollow liner 68 within liquefiertube 64, particularly the location of outlet end 84, divides liquefierassembly 62 into upstream portion 62 a and downstream portion 62 b.Outlet end 84 of hollow liner 68 is located within liquefier tube 64,between inlet end 70 and outlet end 72, such that extrusion tip 66 isdisposed at an offset location from outlet end 84 along longitudinallength 74. Outlet end 84 is desirably positioned along liquefier tube 64at a location at which a desired meniscus of the molten material will belocated, which may vary depending on the operating parameters ofliquefier assembly 62, such as the dimensions of liquefier assembly 62,the material and geometry of the intended filament, the thermal gradientalong liquefier tube 64, and the like. In the embodiment shown in FIG.3, outlet end 84 is located above zone 78.

Hollow liner 68 may be fabricated from a variety of different materialsthat are capable of withstanding the thermal environment of buildchamber 12 (shown in FIG. 1) and liquefier assembly 62 without meltingor thermal degradation for a suitable operational life. Suitablematerials for hollow liner 68 include fluorinated polymers (e.g.,perfluoropolymers), diamond-like carbon materials, graphite, ceramicalloys, metallic materials (e.g., stainless steel), and combinationsthereof.

In some embodiments, inner surface 86 of hollow liner 68 and upstreamportion 62 a has a low surface energy. For example, hollow liner 68 maybe fabricated from one or more materials having low coefficients offriction, such as fluorinated polymers (e.g., perfluoropolymers),diamond-like carbon materials, ceramic alloys, and combinations thereof.In this embodiment, suitable materials for hollow liner 68 includematerials having static coefficients of friction less than about 0.3,with particularly suitable materials having static coefficients offriction less than about 0.2, and with even more particularly suitablematerials having static coefficients of friction less than about 0.1,where static coefficient of frictions referred to herein are measuredpursuant to ASTM D1894-08.

Examples of suitable fluorinated polymers includepolytetrafluoroethylenes (PTFE), fluorinated ethylene propylenes, andperfluoroalkoxy polymers. Examples of suitable commercially availablefluorinated polymers include PTFE available under the trade designation“TEFLON” from E.I. du Pont de Nemours and Company, Wilmington, Del.Examples of suitable ceramic alloys include those based onaluminum-magnesium-boride (AlMgB₁₄), such as AlMgB₁₄ alloys withtitanium boride (TiB₂).

Additionally or alternatively, the low surface energy may be attainedthrough one or more surface modification techniques (e.g., polishing).Accordingly, suitable surface energies for inner surface 86 includesurface energies less than about 75 millinewtons/meter, withparticularly suitable surface energies including less than about 50millinewtons/meter, and with even more particularly suitable surfaceenergies including less than about 25 millinewtons/meter, where thesurface free energies referred to herein as measured pursuant to ASTMD7490-08.

As discussed below, a low surface energy allows hollow liner 68 (andupstream portion 62 a) to have a small inner diameter, thereby providinga small gap between inner surface 86 and a filament used with liquefierassembly 62. In the shown embodiment, hollow liner 68 has asubstantially uniform inner diameter at inner surface 86, referred to asinner diameter 86 d, which is also the inner diameter of upstreamportion 62 a.

Suitable diameters for inner diameter 86 d may vary depending on theaverage diameter of the intended filament. Alternatively, as discussedbelow, hollow liner 68 may be compliant such that inner diameter 86 dmay tend to size onto the filament. Examples of suitable averagediameters for inner diameter 86 d range from about 1.32 millimeters(about 0.052 inches) to about 3.10 millimeters (about 0.122 inches). Insome embodiments, examples of suitable average diameters for innerdiameter 86 d range from about 101% to about 110% of the averagediameter of the intended filament, with particularly suitable averagediameters ranging from about 102% to about 105% of the average diameterof the intended filament.

In the shown embodiment, hollow liner 68 also has a substantiallyuniform wall thickness, referred to as wall thickness 68 t. Examples ofsuitable average thicknesses for wall thickness 68 t range from about0.25 millimeters (about 0.01 inches) to about 2.0 millimeters (about0.08 inches), with particularly suitable thicknesses ranging from about0.33 millimeters (about 0.13 inches) to about 0.48 millimeters (about0.02 inches). As shown, wall thickness 68 t at outlet end 84 definesshoulder 88, which is a downward-facing annular surface that offsets thedistance between inner diameters 76 d and 86 d, thereby providing thestepped cross-sectional areas.

Suitable lengths for hollow liner 68 along longitudinal axis 74 may alsovary depending on the operating parameters, and on the length ofliquefier tube 64. The height shoulder 88 along longitudinal axis 74relative to outlet end 72 of liquefier tube 64 is desirably positionedalong liquefier tube 64 at a height at which a desired meniscus of themolten material is to be located. Examples of suitable average distancesfor shoulder 88 to be located from outlet end 72, referred to as height88 h, range from about 13 millimeters (about 0.5 inches) to about 130millimeters (about 5.0 inches), with particularly suitable distancesranging from about 25 millimeters (about 1.0 inch) to about 51millimeters (about 2.0 inches).

Liquefier assembly 62 may be manufactured using conventional techniques.For example, liquefier tube 64 may be cast, extruded, drawn, orotherwise molded to desired dimensions. Hollow liner 64 may also becast, extruded, drawn, or otherwise molded to desired dimensions, andthen inserted within liquefier tube 64. Hollow liner 68 may be fixedlysecured within liquefier tube 64, or may be loosely retained withinliquefier tube 64, depending on the particular arrangements of hollowliner 68. For example, hollow liner 68 may be retained within liquefiertube 64 with a mild frictional fit, while a top end of hollow liner 68(at inlet end 82) may be folded over, outside of liquefier tube 64 toprevent hollow liner 68 from sliding downward into liquefier tube 64.Extrusion tip 66 may also be secured to liquefier tube 64 and outlet end72, as discussed above, or otherwise disposed at outlet end 72.

FIG. 4 shows liquefier assembly 62 in use with filament 90, wherereference labels for wall thicknesses 64 t and 68 t, for inner diameters76 d and 86 d, and for height 88 h are omitted for ease of viewing anddiscussion. As shown in FIG. 4, during a build operation with filament90, successive solid segments of filament 90 are fed into inlet end 82of hollow liner 68, passed through the lumen region of hollow liner 68,and out of outlet end 84 into zone 78 of liquefier tube 64. Thesuccessive solid segments are then melted in zone 78 to provide moltenmaterial, referred to as melt 92. Melt 92 forms or otherwise definesmeniscus 94 around the unmelted portion of filament 90, where the upwardmovement of meniscus 94 is substantially blocked by shoulder 88 ofhollow liner 68.

Filament 90 includes outer surface 95. As shown, the gap between innersurface 86 of hollow liner 68/upstream portion 62 a and outer surface 95of filament 90, referred to as gap 96, is small compared to the gapbetween inner surface 76 of liquefier tube 64/downstream portion 62 band outer surface 95, referred to as gap 98. Gap 96 is measured as halfof the difference between the average diameter of inner diameter 86 d(shown in FIG. 3) and the average diameter of filament 90 in a solidstate, along longitudinal length 74. Similarly, gap 98 is determined ashalf of the difference between the average diameter of inner diameter 76d (shown in FIG. 3) and the average diameter of filament 90 in a solidstate, along longitudinal length 74.

Accordingly, upstream portion 62 a, as defined by hollow liner 64,allows the combination of a narrow upper gap 96 and a larger lower gap98 to be attained in a single liquefier. The larger dimensions for gap98 allow a larger volume of melt 102 to exist in zone 78, which mayincrease the heat transfer rate within zone 78 and may reduce meniscusdry down effects. The smaller dimensions for gap 96 provide a relativelytight fit between hollow liner 68 and filament 90, thereby preventingmeniscus 94 from rising within liquefier tube 64. This is in addition toshoulder 88, which itself physically blocks meniscus 94 from rising.Furthermore, in some embodiments, the low surface energy of innersurface 86 may also prevent the molten material of meniscus 94 fromsticking to inner surface 86 when solidifying within zone 80.

During a steady-state extrusion of the material through extrusion tip66, the height of meniscus 94 is maintained at the height of shoulder 88along longitudinal length 74. The downward movement of filament 90functions as a viscosity pump to extrude the material in melt 92 out ofextrusion tip as extruded road 99. The hydraulic pressure required toextrude the material out of extrusion tip 66 is generated by shearingthe flow of the molten material within a few millimeters of meniscus 94using the viscosity pump action.

When the feed rate of filament 90 is increased, meniscus 94 does notexhibit a substantial dry down effect. In other words, the height ofmeniscus 94 is substantially maintained at or adjacent to shoulder 88.This is in comparison to meniscus 52 of liquefier assembly 38 shown inFIG. 2B, which may exhibit a significant dry down effect when the feedrate of filament 40 is increased.

While not wishing to be bound by theory, it is believed that thereduction in the meniscus dry down may be due to a combination ofdissimilar thermal conductivities between liquefier tube 64 and hollowliner 68, and the large dimensions of gap 98 compared to gap 96. Thecombination of the dissimilar thermal conductivities between liquefiertube 64 and hollow liner 68 provides a defined and abrupt thermalgradient at shoulder 88. This thermal gradient is believed to assist inreducing the dry down of meniscus 94 when the feed rate of filament 90is increased.

Additionally, the larger dimensions for gap 98 reduce the pressuregenerated by the viscosity pump action within zone 78. This also reducesthe dry down of meniscus 94 when the feed rate of filament 90 isincreased. As such, even when the feed rate of filament 90 is increased,the effective heated length of liquefier assembly 62 is substantiallymaintained at shoulder 88. This constant heated length allows liquefierassembly 62 to achieve higher flow rates than the peak flow rateattainable with a liquefier assembly 38 having the same dimensions.

In the event that liquefier assembly 62, while operating at asteady-state extrusion rate at a modest speed, is suddenly directed tostop extruding (corresponding to liquefier assembly 38 shown above inFIG. 2C), latent heat may exist to melt filament 90 in zone 78. However,shoulder 88, the small dimensions of gap 96, and the low surface energyof inner surface 86 restrict or prevent meniscus 94 from rising upwardinto zone 80 of liquefier tube 64. This reduces or prevents moltenfilament material from reaching zone 80, which may otherwise cool downand effectively plug of liquefier assembly 62, as discussed above forliquefier assembly 38 in FIG. 2C.

In some embodiments, hollow liner 68 may be compliant such that smallamounts of the molten filament material of melt 92 may creep up orotherwise flow between liquefier tube 64 and hollow liner 68 at shoulder88. In these embodiments, hollow liner 68 desirably exhibits a Young'sModulus ranging from about 0.1 gigapascals to about 5.0 gigapascals,with particularly suitable Young's Modulus values ranging from about 0.2gigapascals to about 3.0 gigapascals, and with even more particularlysuitable Young's Modulus values ranging from about 0.3 gigapascals toabout 1.0 gigapascal, where Young's Modulus values referred to hereinare measured pursuant to ASTM D4065-06. In comparison, suitable Young'sModulus values for liquefier tube 64 include values of at least about 50gigapascals, with particularly suitable Young's Modulus values includingvalues of at least about 100 gigapascals.

For example, hollow liner 68 derived from a PTFE tubing (Young's Modulusof about 0.5 gigapascals) may exhibit this form of compliance in astainless-steel liquefier tube 64 (Young's Modulus of about 200gigapascals). The small of amounts of material tend size inner diameter86 d (shown in FIG. 3) onto filament 90, such that inner surface 86tends to contact various portions of outer surface 95 of filament 90(i.e., gap 96 is reduced). As such, the thickness of meniscus 94 at gap96 becomes small enough such that the viscosity pump action effectivelybecomes fixed. This renders the viscosity pump action in liquefierassembly 62 independent of variations in the diameter of filament 90. Asa result, the height of meniscus 94 does not substantially change due tovariations in the diameter of filament 90.

As discussed below, in alternative embodiments, liquefier assembly 62may exhibit a non-cylindrical geometry (e.g., a rectangular geometry).In these embodiments, diameters 76 d and 86 d may alternatively bemeasured and compared based on their respective inner cross-sectionalareas. For example, an inner diameter 76 d of 2.03 millimeters (0.080inches) provides a hollow cross-sectional area of 3.24 squaremillimeters (0.005 square inches), and an inner diameter 86 d of 1.83millimeters (0.072 inches) provides an inner cross-sectional area of2.63 square millimeters (0.004 square inches). Thus, in this example,the inner cross-sectional area of inner surface 76 is about 123% of theinner cross-sectional area of inner surface 86.

Accordingly, since inner diameter 76 d of downstream portion 62 b isgreater than inner diameter 86 d of upstream portion 62 a, the averageinner cross-sectional area corresponding to inner diameter 76 d is alsogreater than the average inner cross-sectional area corresponding toinner diameter 86 d. Examples of suitable inner cross-sectional areasfor inner surface 76 relative to inner surface 86 includecross-sectional areas of at least about 105% of the innercross-sectional area of inner surface 86, with particularly suitablecross-sectional areas ranging from about 110% to about 150% of the innercross-sectional area of inner surface 86, and with even moreparticularly suitable cross-sectional areas ranging from about 115% toabout 135% of the inner cross-sectional area of inner surface 86.

The use of a stepped cross-sectional area, such as with hollow liner 68within liquefier tube 64, restricts movement of meniscus 94 at a desiredheight within liquefier tube 64. For example, hollow liner 68 mayachieve a combination of reducing the dry down of meniscus 94, blockingmeniscus 94 from flowing upward due to thermal expansion, andcompensating for diameter variations in filament 90. This improvescontrol of extrusion rates and response times for building 3D models andsupport structures (e.g., 3D model 24 and support structure 26) havinggood part quality and reduced build times.

FIGS. 5-11 illustrate alternatives to liquefier assembly 62 (shown inFIGS. 3 and 4) having corresponding upstream and downstream portions,where corresponding reference labels are increased by “100”, “200”,“300”, “400”, “500”, and “600”, respectively. Heating coilscorresponding to heating coils 77 (shown in FIGS. 3 and 4) are omittedfor ease of discussion. As shown in FIG. 5, liquefier assembly 162includes hollow liner 168 having inner coating 168 a and outer portion168 b, where inner coating 168 a is disposed on an inner surface ofouter portion 168 b. In this embodiment, inner coating 168 a maycompositionally include one or more materials having low coefficients offriction and/or that provide low surface energies for inner surface 186,and that are capable of withstanding the thermal environment of buildchamber 12 (shown in FIG. 1) and liquefier assembly 162 without meltingor thermal degradation for a suitable operational life. Examples ofsuitable materials include those discussed above for hollow liner 68.

Outer portion 168 b may compositionally include a different materialfrom inner coating 168 a. For example, outer portion 168 b may bederived from one or more materials having low thermal conductivities,and that are capable of withstanding the thermal environment of buildchamber 12 (shown in FIG. 1) and liquefier assembly 162 without meltingor thermal degradation for a suitable operational life. Accordingly,hollow liner 168 may be fabricated with different materials to attaindifferent physical and thermal properties. For example, inner coating168 a may be derived from a first material having a low coefficient offriction, but that does not necessarily have a low thermal conductivity;and outer portion 168 b may be derived from a second material having alow thermal conductivity, but that does not necessarily have a lowcoefficient of friction.

In the shown embodiment, outer portion 168 b constitutes the bulk ofhollow liner 168, and inner coating 168 a constitutes a thin layerapplied to the inner surface of outer portion 168 b. This is suitablefor embodiments in which the thermal properties of outer portion 168 bare desired (e.g., low thermal conductivity), and in which the surfaceproperties of inner coating 168 a are desired (e.g., low surfaceenergy). In alternative embodiments, hollow liner 168 may include threeor more layers, where the thickness of each layer may vary depending onthe desired properties to be attained. In yet another embodiment, outerportion 168 b may be integrally formed with liquefier tube 164 as asingle component, and inner coating 168 a may then be applied to theinner surface of outer portion 168 b.

As shown in FIG. 6, liquefier assembly 262 includes liquefier tube 264and extrusion tip 266, where liquefier tube 264 has a wall thickness 264t that is thinner than wall thickness 64 t of liquefier tube 64 (shownin FIG. 3). Additionally, liquefier assembly 262 also includes thermalsheath 300, which, in the shown embodiment, is integrally formed withextrusion tip 266 and extends around liquefier tube 264 beyond theextent covered by extrusion tip 66 (shown in FIGS. 3 and 4). This allowsextrusion tip 266 to laterally support liquefier tube 264 from thehydraulic pressures exerted by the viscosity pump action, in addition toproviding good thermal spreading and heat control.

Thermal sheath 300 extends around liquefier tube 264 beyond the heightof outlet end 284 of hollow liner 268. In alternative embodiments,thermal sheath 300 may extend around liquefier tube 264 up to differentheights along longitudinal length 274, where the extent desirablyreaches a height that is at least as far up along longitudinal length274 as outlet end 284 of hollow liner 268.

Wall thickness 264 t may be less than a thickness that would otherwisebe required for an unsupported liquefier tube. Examples of suitablethicknesses for wall thickness 264 t range from about 0.05 millimeters(about 0.002 inches) to about 0.25 millimeters (about 0.010 inches),with particularly suitable thicknesses ranging from about 0.076millimeters (about 0.003 inches) to about 0.15 millimeters (about 0.006inches). The reduced wall thickness for liquefier tube 264 is beneficialfor reducing the thermal conductivity vertically along longitudinallength 274. Additionally, the reduced wall thickness reduces the mass ofliquefier tube 264, thereby reducing the time required to heat liquefiertube 264 from an ambient temperature to an operating temperature.

As shown in FIG. 7, liquefier assembly 362 includes liquefier tube 364,extrusion tip 366, and thermal sheath 400, which may function in thesame manner as liquefier tube 264, extrusion tip 266, and thermal sheath300 (shown in FIG. 6). Liquefier assembly 362 also includes lower tube402, which extends around inner surface 376 of liquefier tube 362 atzone 378. Liquefier assembly 362 (and the other liquefier assemblies ofthe present disclosure) desirably operates with a thermal profile thatreduces the risk of, or eliminates, thermal degradation of the filamentmaterials. However, in some situations, gas may be generated in melt392, typically from thermal degradation of some filament materialsand/or the generation of steam from wet filament materials. Thisgenerated gas may collect in the annulus defined by the shoulder of thehollow liner (e.g., shoulder 88 of liquefier assembly 62, shown in FIGS.3 and 4), which can result in oozing of the extruded material at theends of the extruded roads (e.g., extruded road 99, shown in FIG. 4).

To reduce or prevent build up of such generated gas, lower tube 402 maybe disposed within liquefier tube 364 to reduce the thickness of theannulus defined by shoulder 388. Lower tube 402 is desirably fabricatedfrom one or more materials having high thermal conductivities, such asstainless steel, to assist in the heat transfer to filament 390 withinzone 378. As shown, the use of lower tube 402 reduces the thickness ofthe annulus created by shoulder 388 such that the viscosity pump actioncan carry any generated gas pockets down through extrusion tip 366.

As shown in FIG. 8, liquefier assembly 462 includes liquefier tube 464,extrusion tip 466, and thermal sheath 500, which may function in thesame manner as liquefier tube 264, extrusion tip 266, and thermal sheath300 (shown in FIG. 6). Liquefier assembly 462 also includes hollow liner468 and rotary drive nut 504. In this embodiment, hollow liner 468includes threaded portion 506 adjacent to inlet end 482, and may beadjusted in height along longitudinal length 474 relative to liquefiertube 464. As shown, threaded portion 506 is engaged with rotary drivenut 504, which allows the height of hollow liner 468 to be adjustedrelative to liquefier tube 464 by rotation of rotary drive nut 504.

The adjustment mechanism for hollow liner 468 accordingly adjusts theheight of shoulder 488, thereby allowing the effective heated length ofliquefier assembly 462 to be adjusted. This may be used for a variety ofpurposes, such as modifications to the response times for liquefierassembly 462, gas purging operations, and seam control in the 3D modelsand support structures (e.g., 3D model 24 and support structure 26).

With respect to modifications to the response times for liquefierassembly 462 while operating at steady state, liquefier assembly 462 hasits maximum flow rate dictated by its heated length and the thermaldiffusivity of the filament material being extruded, where the heatedlength is effectively determined by the height of meniscus 494. However,the small orifice diameter of extrusion tip 466 provides a flowresistance that is large compared to the flow resistance withinliquefier tube 464. As a result, liquefier assembly 462 has a responsetime that increases with the square of the heated length of liquefiertube (i.e., the height of meniscus 494).

Therefore, the longer the heated length, the more difficult it may be tochange the flow rate quickly (i.e., a slower response time). This slowerresponse time accordingly slows down the build speed when building 3Dmodels and support structures, particularly in regions in which numerousstops and starts are required. Stated another way, when an extrusionhead (e.g., extrusion head 18) carrying liquefier assembly 462 movesquickly through interior fill patterns of a 3D model or supportstructure, a longer heated length is desired. Alternatively, when theextrusion head traces surface details with numerous stops and starts, ashorter heated length is desired. Accordingly, a long heated length ofliquefier assembly 462 (i.e., a high shoulder 488 and meniscus 494)provides a high flow rate and a slow time response, while a short heatedportion of liquefier assembly 462 (i.e., a low shoulder 488 and meniscus494) provides a low flow rate and a fast time response.

Accordingly, by adjusting the height of shoulder 488 along longitudinallength 474 relative to liquefier tube 464 and extrusion tip 466, theeffective heated length of liquefier assembly 462 may be adjusted toaccommodate different aspects of a build operation. When an interiorfill pattern is being built, rotary drive nut 504 may rotate and raisehollow liner 468 such that shoulder 488 is positioned at a desiredheight to retain meniscus 494. This results in a long heated length ofliquefier assembly 462, which slows the response time, but providesfaster flow rates.

Alternatively, when exterior surface details are being built, rotarydrive nut 504 may rotate in the opposing direction and lower hollowliner 468 such that shoulder 488 is positioned at a desired loweredheight to retain meniscus 494. This results in a shorter heated lengthof liquefier assembly 462, which slows the flow rates, but improves theresponse time for building the surface features. In an alternativeembodiment, the effective heated length may also (or alternatively) beadjusted with the use of a multiple-zone liquefier arrangement, asdisclosed in Swanson et al., U.S. patent application Ser. No.12/841,341, entitled “Multiple-Zone Liquefier Assembly forExtrusion-Based Additive Manufacturing Systems”.

While illustrated with rotary drive nut 504, the height of hollow liner468 may be adjusted using a variety of different mechanisms. In oneembodiment, the height of hollow liner 468 may be adjusted to functionas a secondary pump to increase the extrusion rate of melt 492 throughextrusion tip 466 for a short duration. For example, hollow liner 468may be quickly lowered and raised back up to create a short pumpingaction. This is suitable for use in a variety of applications, such asfor providing an initial spike in pumping action at the start point of atool path.

In another alternative embodiment, rotary drive nut 504 may be replacedwith a force gauge (not shown) that is operably secured to hollow liner468 at inlet end 582. The force gauge may monitor the upward pressureapplied to hollow liner 468, thereby measuring the pressure of melt 592within liquefier assembly 562. Accordingly, the movement and complianceof hollow liner 568 provides a variety of functional andmeasurement-based operations that may be performed with liquefierassembly 562.

As shown in FIG. 9, liquefier assembly 562 includes liquefier tube 564,extrusion tip 566, hollow liner 568, and thermal sheath 600, which mayfunction in the same manner as liquefier tube 264, extrusion tip 266,hollow liner 268, and thermal sheath 300 (shown in FIG. 6). Liquefierassembly 562 also includes sensor 608 and electrical connection 609,where sensor is retained between liquefier tube 564 and hollow liner 568at shoulder 588. Electrical connection 609 is connected to sensor 608,and extends upward between liquefier tube 564 and hollow liner 568, andout of inlet end 570. As such, electrical connection 609 interconnectssensor 608 with an external electronic unit (not shown).

Sensor 608 may be any suitable sensor for monitoring thermal, flow, andpressure properties within liquefier assembly 562. For example, sensor608 may be a thermocouple and/or a pressure transducer for measuringtemperatures, pressures, and flow rates of melt 592. This embodiment isparticularly suitable for use with hollow liner 568 derived from amaterial that is compliant, as discussed above. Retaining sensor 608between liquefier tube 564 and hollow liner 568 allow real-timemonitoring of melt 592, where the monitored properties may be relayedback to controller 28 to control operation of extrusion head 18.

While illustrated with a single sensor 608, the liquefier assemblies ofthe present disclosure may alternatively include multiple sensors 608.In an additional alternative embodiment, hollow liner 568 may include anadditional lumen extending from inlet end 582 to outlet end 584, runningparallel to the inner lumen of hollow liner 568. One or more sensors 608and electrical connections 609 may then extend through the additionallumen, rather than between liquefier tube 564 and hollow liner 568.

As shown in FIG. 10, liquefier assembly 662 includes liquefier tube 664,extrusion tip 666, and thermal sheath 700, which may function in thesame manner as liquefier tube 264, extrusion tip 266, and thermal sheath300 (shown in FIG. 6). Liquefier assembly 662 also includes hollow liner668 having sloped shoulder 688.

As discussed above, in some situations, gas may be generated in melt692, typically from thermal degradation of some filament materials. Thisgenerated gas may collect in the annulus defined by the shoulder of thehollow liner (e.g., shoulder 88 of liquefier assembly 62, shown in FIGS.3 and 4), which can result in oozing of the extruded material at theends of the extruded roads (e.g., extruded road 99, shown in FIG. 4).

As shown in FIG. 11, to reduce or prevent build up of such generatedgas, shoulder 688 exhibits a sloped profile relative to the crosssection of liquefier assembly 662, where the cross section of liquefierassembly 662 is taken perpendicular to longitudinal axis 674. The slopedprofile of shoulder 688 allows any generated gas to pass upward throughupper portion 662 a, and out of inlet end 682. While illustrated with aparticular sloped profile, shoulder 688 may alternatively includedifferent slope angles and/or curved profiles to assist in degassingliquefier assembly 662.

FIGS. 12A-12D illustrate suitable cross-sectional geometries for hollowliner 68, where, in these embodiments, the outer surface of hollow liner68 retains a circular geometry (shown in FIGS. 3 and 4). While thefollowing discussion of the suitable cross-sectional geometries is madewith reference to hollow liner 68, the suitable cross-sectionalgeometries are also applicable to any hollow liner of the presentdisclosure (e.g., hollow liners 68, 168, 268, 368, 468, 568, and 668).FIG. 12A is a sectional view of section 12A-12A taken in FIG. 3,illustrating an annular cross-sectional geometry for use with acylindrical filament (e.g., filament 90), which provides a circularhollow cross-sectional area for inner surface 86. However, as shown inFIGS. 12B-12D, hollow liners having non-annular cross-sectionalgeometries may be used with liquefier tube 64 (and/or liquefier tubes164, 264, 364, 464, 564, and 664) to receive a variety of differentfilament cross-sectional shapes.

As shown in FIG. 12B, hollow liner 768 includes a pair of rectangularapertures 768 a and 768 b, which are each configured to receive a ribbonfilament. Examples of suitable ribbon filaments for use in thisembodiment include those disclosed in Batchelder et al., U.S. patentapplication Ser. No. 12/612,333, entitled “Non-Cylindrical Filaments foruse in Extrusion-Based Digital Manufacturing Systems”. Since themeniscus has little tendency to climb upward, one of the pair of ribbonfilaments may be fed while the other remains stationary (e.g., for apair of modeling and support materials). While illustrated with a pairof rectangular apertures 768 a and 768 b, hollow liner 768 mayalternatively include a single rectangular aperture, or three or morerectangular apertures.

As shown in FIG. 12C, hollow liner 868 includes three apertures 868a-868 c, each of which are configured to receive a cylindrical filament.This embodiment is suitable for feeding filaments of different materials(e.g., different colors) into the liquefier assembly, which may then bemelted and at least partially blended together before extrusion.Examples of suitable design arrangements and materials for use in thisembodiment include those disclosed in Zinniel et al., U.S. patentapplication Ser. No. 12/820,370, entitled “Consumable Materials havingCustomized Characteristics”.

As shown in FIG. 12D, hollow liner 968 includes an off-axis aperture 968a, which may improve the tendency of the filament to compress into aspiral near inner surface 76 in heated zone 78 as it melts, therebyincreasing the heat transfer rate. Axially asymmetric feed can encouragehelical flow in the heated portion.

As discussed above, in some embodiments, the hollow liner may include anadditional lumen for extending sensors through the hollow liner. Theoff-axis geometry of hollow liner 968 is suitable for providing such anadditional lumen, such as aperture 968 b (shown with dashed lines). Assuch, one or more sensors and electrical connections may then extendthrough aperture 968 b to monitor various thermal, flow, andpressure-based properties.

FIG. 13 illustrates liquefier assembly 1062, which is an additionalalternative to liquefier assembly 62 (shown in FIGS. 3 and 4), wherecorresponding reference labels are increased by “1000”. As shown in FIG.13, liquefier assembly 1062 includes liquefier tube 1064, extrusion tip1066, and thermal sheath 1100, which may function in a similar manner asliquefier tube 264, extrusion tip 266, and thermal sheath 300 (shown inFIG. 6). In this embodiment, however, liquefier tube 1064 itself definesupstream portion 1062 a, and liquefier sheath 1100 defines downstreamportion 1062 b. Suitable materials for liquefier tube 1064 include thosediscussed above for liquefier tube 64 (shown in FIGS. 3 and 4), such asgraphite.

Suitable inner diameters for liquefier tube 1064 and upstream portion1062 a include those discussed above for inner diameter 86 d of hollowliner 68 (shown in FIG. 3). Correspondingly, suitable inner diametersfor thermal sheath 1100 and downstream portion 1062 b include thosediscussed above for inner diameter 76 d of liquefier tube 64 (shown inFIG. 3). The use of thermal sheath 1100 allows extrusion tip 1066 tolaterally support liquefier tube 1064 from the hydraulic pressuresexerted by the viscosity pump action, in addition to providing goodthermal spreading and heat control. Accordingly, liquefier assembly 1062is suitable for use in high-temperature environments, such as those atwhich a fluoropolymer hollow liner may not be thermally stable.

The above-discussed embodiments of the liquefier assemblies of thepresent disclosure are described as having cylindrical geometries. Asdiscussed above, in alternative embodiments, liquefier assemblies 62,162, 262, 362, 462, 562, 662, and 1062 may exhibit non-cylindricalgeometries (e.g., rectangular geometries). Examples of suitablegeometries for liquefier assemblies 62, 162, 262, 362, 462, 562, 662,and 1062 include those disclosed in Batchelder et al., U.S. patentapplication Ser. No. 12/612,329, entitled “Ribbon Liquefier for use inExtrusion-Based Digital Manufacturing Systems”; and Batchelder et al.,U.S. patent application Ser. No. 12/612,333, entitled “Non-CylindricalFilaments for use in Extrusion-Based Digital Manufacturing Systems”.

In these embodiments, the above-discussed “diameters” for the liquefiertubes (e.g., liquefier tube 64, 164, 264, 364, 462, 562, 662, and 1062),the hollow liners (e.g., hollow liners 68, 168, 268, 368, 468, 568, and1068), the filaments (e.g., filaments 90, 190, 290, 390, 490, 590, 690,and 1090) may be replaced with corresponding non-cylindrical geometriesbased on the respective cross-sectional areas. As discussed above, theaverage inner cross-sectional area for a liquefier assembly upstreamportion (e.g., upstream portions 62 a, 162 a, 262 a, 362 a, 462 a, 562a, 662 a, and 1062 a) is greater than the average inner cross-sectionalarea for the corresponding liquefier assembly downstream portion (e.g.,downstream portions 62 b, 162 b, 262 b, 362 b, 462 b, 562 b, 662 b, and1062 b).

Furthermore, the features of the above-discussed embodiments may becombined to attain additional alternative embodiments. For example, theinner coating embodiment of hollow liner 168 (shown in FIG. 5) may alsobe used in combination with liquefier assemblies 262, 362, 462, 562,662, and 1062; lower tube 402 (shown in FIG. 7) may also be used incombination with liquefier assemblies 62, 162, 262, 462, 562, 662, and1062; the hollow liner adjustment mechanism (e.g., rotary drive nut 504,shown in FIG. 8) may also be used in combination with liquefierassemblies 62 and 162; the sensor-containing embodiment of liquefierassembly 562 (shown in FIG. 9) may also be used in combination withliquefier assemblies 62 and 162; the sloped-shoulder embodiment ofliquefier assembly 662 (shown in FIGS. 10 and 11) may also be used incombination with liquefier assemblies 62 and 162; and combinationsthereof.

The consumable materials (e.g., filaments) for use in the liquefierassemblies of the present disclosure (e.g., liquefier assemblies 62,162, 262, 362, 462, 562, 662, and 1062) may include a variety ofextrudable modeling and support materials for respectively building 3Dmodel 24 and support structure 26. Suitable modeling materials for usewith the lined liquefier assemblies of the present disclosure includepolymeric and metallic materials. In some embodiments, suitable modelingmaterials include materials having amorphous properties, such asthermoplastic materials, amorphous metallic materials, and combinationsthereof. Examples of suitable thermoplastic materials for the consumablematerials include acrylonitrile-butadiene-styrene (ABS) copolymers,polycarbonates, polysulfones, polyethersulfones, polyphenylsulfones,polyetherimides, amorphous polyamides, modified variations thereof(e.g., ABS-M30 copolymers), polystyrene, and blends thereof. Examples ofsuitable amorphous metallic materials include those disclosed inBatchelder, U.S. Patent Application Publication No. 2009/0263582.

Suitable support materials for use with the liquefier assemblies of thepresent disclosure include polymeric materials. In some embodiments,suitable support materials include materials having amorphous properties(e.g., thermoplastic materials) and that are desirably removable fromthe corresponding modeling materials after 3D model 28 and supportstructure 30 are built. Examples of suitable support materials for thefilaments include water-soluble support materials commercially availableunder the trade designations “WATERWORKS” and “SOLUBLE SUPPORTS” fromStratasys, Inc., Eden Prairie, Minn.; break-away support materialscommercially available under the trade designation “BASS” fromStratasys, Inc., Eden Prairie, Minn., and those disclosed in Crump etal., U.S. Pat. No. 5,503,785; Lombardi et al., U.S. Pat. Nos. 6,070,107and 6,228,923; Priedeman et al., U.S. Pat. No. 6,790,403; and Hopkins etal., U.S. Patent Application Publication No. 2010/0096072.

The compositions of the modeling and support materials may also includeadditional additives, such as plasticizers, rheology modifiers, inertfillers, colorants, stabilizers, and combinations thereof. Examples ofsuitable additional plasticizers include dialkyl phthalates, cycloalkylphthalates, benzyl and aryl phthalates, alkoxy phthalates, alkyl/arylphosphates, polyglycol esters, adipate esters, citrate esters, esters ofglycerin, and combinations thereof. Examples of suitable inert fillersinclude calcium carbonate, magnesium carbonate, glass spheres, graphite,carbon black, carbon fiber, glass fiber, talc, wollastonite, mica,alumina, silica, kaolin, silicon carbide, composite materials (e.g.,spherical and filamentary composite materials), and combinationsthereof. In embodiments in which the composition includes additionaladditives, examples of suitable combined concentrations of theadditional additives in the composition range from about 1% by weight toabout 10% by weight, with particularly suitable concentrations rangingfrom about 1% by weight to about 5% by weight, based on the entireweight of the composition.

Although the present disclosure has been described with reference toseveral embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the disclosure.

1. A liquefier assembly for use in an extrusion-based additivemanufacturing system, the liquefier assembly comprising: a downstreamportion comprising a first end and a second end opposite of the firstend along a longitudinal length of the liquefier assembly, wherein thedownstream portion has a first average inner cross-sectional area; anupstream portion disposed adjacent to the first end of the downstreamportion, the upstream portion being configured to receive a consumablematerial, wherein the upstream portion has a second average innercross-sectional area that is less than the first inner cross-sectionalarea, and wherein the upstream portion defines a shoulder between thefirst inner cross-sectional area and the second inner cross-sectionalarea, the shoulder being configured to restrict movement of a meltmeniscus of the consumable material; and an extrusion tip disposed atthe second end of the downstream portion.
 2. The liquefier assembly ofclaim 1, wherein the upstream portion comprises an inner surface havinga surface energy less than about 75 millinewtons/meter.
 3. The liquefierassembly of claim 1, wherein the first average inner cross-sectionalarea is at least 105% of the second average inner cross-sectional area.4. The liquefier assembly of claim 3, wherein the first average innercross-sectional area ranges from about 110% of the second average innercross-sectional area to about 150% of the second average innercross-sectional area.
 5. The liquefier assembly of claim 1, wherein theupstream portion comprises: a liquefier tube; and a hollow linerdisposed in the liquefier tube.
 6. The liquefier assembly of claim 5,and further comprising at least one sensor disposed between theliquefier tube and the hollow liner.
 7. A liquefier assembly for use inan extrusion-based additive manufacturing system, the liquefier assemblycomprising: a liquefier tube having an inlet end, an outlet end, and aninner surface, wherein the inner surface of the liquefier tube defines afirst average inner cross-sectional area; a hollow liner disposed atleast partially within the liquefier tube such that an outlet end of thehollow liner is disposed within the liquefier tube at a location betweenthe inlet end of the liquefier tube and the outlet end of the liquefiertube, wherein the hollow liner has an inner surface that defines asecond average inner cross-sectional area, and wherein the first averageinner cross-sectional area is greater than the second average innercross-sectional area to define a shoulder; and an extrusion tip disposedat the outlet end of the liquefier tube at an offset location from theoutlet end of the hollow liner.
 8. The liquefier assembly of claim 7,wherein the inner surface of the hollow liner has a surface energy lessthan about 75 millinewtons/meter.
 9. The liquefier assembly of claim 7,wherein the hollow liner compositionally comprises a material having aYoung's Modulus value ranging from about 0.1 gigapascals to about 5.0gigapascals.
 10. The liquefier assembly of claim 7, wherein the hollowliner has a non-annular cross-sectional geometry.
 11. The liquefierassembly of claim 7, wherein the hollow liner comprises: an innerportion that defines the inner surface of the hollow liner, the innerportion comprising a fluorinated polymer; and an outer portion incontact with at least a portion of the inner surface of the liquefiertube, the outer portion comprising a material that is different from theinner portion.
 12. The liquefier assembly of claim 7, and furthercomprising a mechanism configured to move the hollow liner relative tothe liquefier tube.
 13. The liquefier assembly of claim 7, and furthercomprising at least one sensor disposed between the liquefier tube andthe hollow liner.
 14. The liquefier assembly of claim 7, wherein thehollow liner defines a shoulder at the outlet end of the hollow liner,the shoulder having a sloped profile relative to a cross section of theliquefier assembly, wherein the cross section is taken perpendicular toa longitudinal axis of the liquefier assembly.
 15. The liquefierassembly of claim 7, and further comprising a thermally-conductivesheath extending around a portion of the liquefier tube.
 16. A liquefierassembly for use in an extrusion-based additive manufacturing system,the liquefier assembly comprising: a downstream portion compositionallycomprising a material having an average thermal conductivity greaterthan about 10 watts/meter-Kelvin, wherein the downstream portion has aninner surface with a first average inner cross-sectional area; anupstream portion disposed adjacent to the downstream portion, theupstream portion having an inner surface that has a surface energy lessthan about 75 millinewtons/meter and a second average innercross-sectional area, wherein the first average inner cross-sectionalarea is at least 105% of the second average inner cross-sectional area;and an extrusion tip disposed at an opposing end of the downstreamportion from the upstream portion.
 17. The liquefier assembly of claim16, wherein the surface energy of the inner surface of the upstreamportion is less than about 25 millinewtons/meter.
 18. The liquefierassembly of claim 16, wherein the first average inner cross-sectionalarea ranges from about 110% of the second average inner cross-sectionalarea to about 150% of the second average inner cross-sectional area. 19.The liquefier assembly of claim 16, wherein the upstream portioncomprises a hollow liner that includes the inner surface of the upstreamportion, and wherein the hollow liner compositionally comprises amaterial having a Young's Modulus value ranging from about 0.1gigapascals to about 5.0 gigapascals.
 20. The liquefier assembly ofclaim 16, wherein the inner surface of the upstream portion has anon-cylindrical geometry.